Previous experiments have shown that new mutations, especially spontaneous mutants and to a lesser degree EMS- and radiation-induced, are predominantly small in effect, are partially dominant, cause a decrease in viability, and cause a large decrease in fertility. They are eliminated from the population because of their dominant effects on fitness with a mean persistance of the order of 50 generations. Experiments are being done with inter-species competition in order to try to determine the extent to which selection is based on absolute fitness or is based on some sort of rank-order selection. The phenomenon of hybrid dysgenesis is also being studied in an effort to determine its mode of transmission, its effect on mutation rate, and its likely effect on the population.